Carbon dioxide is not the only gas released by human activities that can cause warming. Human emissions of methane and nitrous oxide together contribute almost half as much warming. The main cause of the hole in the ozone layer is chlorofluorocarbons (CFCs), gases that are used in refrigerators, air conditioners, and industrial applications. While CFCs alone cause warming, their ozone destruction can cause cooling. So far these warming and cooling influences have approximately balanced.